Growing polypeptides that are being secreted by spheroplasts of E. coli have been covalently labeled with a radioactive reagent that cannot penetrate the cell membrane, and their attachment to polysomes has been demonstrated by fractionating the cells and specific proteins labeled in this way is in progress. We will also try to see whether the ribosomes in this fraction have any attachment to the membrane other than that provided by the nascent chain. Puromycin causes release of most of the ribosomes from the membrane fraction, but with cells pretreated with streptomycin many more ribosomes remain with the membrane after puromycin treatment. We are pursuing this observation, in the hope that it may explain the ability of streptomycin to cause membrane damage as well as to inhibit ribosomes. Experiments are continuing on the precise stage in protein synthesis, in the transition between initiation and chain elongation, at which several antibiotics (aminoglycosides, erythromycin, spectinomycin) block the ribosome.